a) Field of the Invention
The present invention relates to an autofocusing device for use with cameras or the like, and more particularly to an autofocusing device of a phase difference detecting type, capable of selectively focusing a desired one of a plurality of subjects.
b) Description of the Related Art
A phase difference detecting type autofocusing method is known as one of a number of autofocusing methods.
According to this method, two images of the single subject are focused by two lenses, and a difference in position of the two focused images changing with the distance from the lenses to the subject, is detected in the form of a phase difference.
FIGS. 7A shows an example of a conventional phase difference detecting type autofocusing device. At the back of an equivalent film plane 52 after a taking lens 51, there are disposed a condenser lens 53, a separator lens 54 and a phase difference detector in this order.
The phase difference detector is constructed of line sensors 55 and 56 such as CCDs and a signal processor 57. The line sensors 55 and 56 photoelectrically convert a pair of images of a subject focussed by the separator lens 54. The signal processor 57 discriminates an in-focus state, based upon electric signals obtained from pixels of the line sensors 55 and 56, the electric signals corresponding to a light intensity distribution on the pixels.
A pair of images focussed on the line sensors 55 and 56 comes near an optical axis 58 if the subject image is of a front focus state before the equivalent film plane 52. Conversely, the pair of images goes away from the optical axis 58 if the subject image is of a rear focus state. At an in-focus state, the pair of images takes an intermediate position between front focus and rear focus.
A phase difference detecting method is used for detecting the positions of images focused on the line sensors 55 and 56. According to this method, an in-focus state is discriminated by a relative shift amount (phase difference) between focused images, the relative shift amount corresponding to a minimum correlation value between pairs of images on the line sensors 55 and 56. A correlation value is given by the following equation (1): EQU H(m)=.SIGMA.(k=1 to n).vertline.B(k)-R(k+m-1).vertline. (1)
where .SIGMA.(k=1 to n) is a sum of functions for k=1 to k=n. k represents a k-th pixel of the line sensor 55 operating as a standard line sensor. m is an integer from 1 to 9 and represents the relative shift amount.
B(k) represents an electric signal time-sequentially outputted from respective pixels of the standard line sensor 55, and R(k+m-1) represents an electric signal time-sequentially outputted from respective pixels of the reference line sensor 56. The equation (1) is calculated for m=1 to 9 to obtain a correlation value H(1), H(2), . . . , H(9).
It is preset that an in-focus state is obtained, for example, when the correlation value H(5) takes a minimum value. If another correlation value takes a minimum value, this shift amount, i.e., a phase difference from m=5, is detected as an out-of-focus.
An example of the structure of the conventional signal processor 57 is shown in FIG. 7B. Electric signals B(k) and R(k) generated at respective pixels of the line sensors 55 and 56 are converted into digital data of 8 bits for example by a microcomputer 60, and temporarily stored in a random access memory (RAM) 61. Thereafter, a central processing unit (CPU) 60 calculates the equation (1) using the digital data stored in RAM 61.
In the autofocusing device shown in FIGS. 7A and 7B, electric charges stored in the photosensors are directly converted into, and detected as, a voltage signal which is then converted into a digital signal and stored in RAM 61, digital signals in RAM 61 being read out and calculated.